The diaphysis (: diaphyses) is the main or midsection (shaft) of a long bone. It is made up of cortical bone and usually contains bone marrow and adipose tissue (fat).
It is a middle tubular part composed of compact bone which surrounds a central marrow cavity which contains red or yellow marrow. In diaphysis, primary ossification occurs.
Ewing sarcoma tends to occur at the diaphysis.